refactor(theme): single-source the HealthChat Light theme

Author the theme once in apps/web/element.io/healthchat/HealthChat-Light-Theme.json
and sync it into the web (apps/web/config.json) and desktop
(apps/desktop/element.io/healthchat/config.json) configs via
scripts/healthchat-sync-theme.mjs, removing the drifted duplicate.
`pnpm healthchat:theme` writes it; `pnpm lint` runs healthchat:theme:check to
fail on drift.

The canonical palette is the reviewed desktop theme, which reverts the web-only
contrast tweak (f64923ea1a) so web and desktop match.

apps/web/package.json

@@ -18,13 +18,15 @@
         "i18n:lint": "matrix-i18n-lint && prettier --log-level=silent --write src/i18n/strings/ --ignore-path /dev/null",
         "i18n:diff": "cp src/i18n/strings/en_EN.json src/i18n/strings/en_EN_orig.json && pnpm i18n && matrix-compare-i18n-files src/i18n/strings/en_EN_orig.json src/i18n/strings/en_EN.json",
         "rethemendex": "sh ./res/css/rethemendex.sh",
+        "healthchat:theme": "node scripts/healthchat-sync-theme.mjs",
+        "healthchat:theme:check": "node scripts/healthchat-sync-theme.mjs --check",
         "build": "nx build",
         "build-stats": "nx build --json=webpack-stats.json",
         "vendor:modernizr": "modernizr -c .modernizr.json -d src/vector/modernizr.js && mv src/vector/modernizr.js src/vector/modernizr.cjs",
         "vendor:jitsi": "curl -s https://meet.element.io/libs/external_api.min.js > ./res/jitsi_external_api.min.js",
         "dist": "./scripts/package.sh",
         "start": "nx start",
-        "lint": "pnpm lint:types && pnpm lint:js && pnpm lint:style",
+        "lint": "pnpm healthchat:theme:check && pnpm lint:types && pnpm lint:js && pnpm lint:style",
         "lint:js": "eslint --max-warnings 0 src test playwright module_system",
         "lint:js-fix": "eslint --fix src test playwright module_system",
         "lint:types": "nx lint:types",